Background: Ethylene glycol (EG) is the chief ingredient in antifreeze solutions. EG ingestion may lead to seizures, coma, acidosis, renal failure and death. Hemodialysis (HD) removes EG, toxic metabolites and corrects acidosis. The use of osmolar gap as a surrogate for serum EG concentration could improve upon currently varied practices and HD run times especially in situations where EG concentrations cannot readily be obtained. We present a case that demonstrates correlation between EG concentration and osmolar gap (OG). Case Report: 47 year-old woman presented to the ED after being found obtunded. She arrived with a pulse of 115 beats/minute, blood pressure 115/81 mmHg, O2 saturation of 100% and Kussmaul respirations. She was intubated and given IV saline. Laboratory values were significant for pH 7.19, anion gap 33 mmol/L, serum osmolality 373 mosm/kg, and a calculated osmolarity of 299 mosm/L. She was given 15 mg/kg fomepizole; her EG level returned at 341 mg/dl. She was placed on HD for 6 hours with resolution of metabolic acidosis and normalization of osmolar and anion gaps. Four EG concentrations were drawn during HD and correlated to the osmolar gaps. The patient was extubated the next day. Her creatinine peaked at 5.3 mg/dL; she required intermittent HD for 10 days and ultimately recovered renal function. Discussion: EG elimination by HD is described by the formula t= [-V ln(5/A)/0.06k]; t is time, V is the Watson estimate of total body water, A is the initial toxin concentration in mmol/L, and k is 80% of the manufacturer specified dialyzer urea clearance. EG concentrations as high as 905 mg/dL have been published making a wide potential range of needed time on HD. It is prudent to continue HD until the anion gap metabolic acidosis resolves and EG concentration is < 20 mg/dl. Practice varies greatly with regard to serial EG concentrations during HD. While the availability of measuring serum EG concentration is limited, the OG is rapidly obtainable at most centers. A published case series highlighting the correlation of OG and serum methanol concentration during HD proposed that using OG to guide HD duration could have reduced HD times. Similar correlation with EG may help prevent over or under utilization of HD and lab resources. A bestfit trend line applied to a scatter plot of the four values each of EG concentrations and OG from the above case correlated with an R2 value of 0.995. Conclusion: We present an EG intoxication where there is strong correlation between osmolar gap and serum EG concentrations during HD. Further study is warranted as use of osmolar gap rather than frequent EG concentrations has the potential to optimize resource utilization and minimize risk to the patient during HD.